Glazing systems: Considerations for the mechanical engineer

Window glazing and shading or louvers have a direct impact on the HVAC load of a building. Mechanical engineers often are tasked with specifying window and shading systems. Know about building envelope, and how it can be managed/altered by window system selection.

Understand how glazing systems can affect a building’s heating and cooling loads.

Learn how to design the building envelope to match the designer’s intent.

Understand the items that can compromise an envelope model.

The abundance of sunlight in buildings is a mixed blessing. In the world of green design, natural light has been found to have enormous positive effects on building occupants. At the same time, the glazing systems that provide this natural light consume a significant amount of the energy required to condition these buildings. With this in mind, effective—and integrated—design and construction of these glazing systems is critical in managing both heating and cooling loads and, perhaps at least as important, infiltration loads in our buildings.

The increasing use of daylighting (the use of natural light in lieu of electric lighting) is an important element of sustainable design in architecture. In addition to the benefits of natural light on the building occupants, managing the operating cost of building lighting systems can have significant positive effects on a building’s energy profile. With building lighting systems alone representing one-third of the average building’s annual energy consumption at the end of the 20th century, reduction in lighting system operating hours can significantly reduce its contribution to building cooling costs.

Finally, creative solutions to managing solar radiation incident on the glazing systems through sun screens leverage the solar heating effect to reduce both heating costs in the winter and cooling costs in the summer.

Given the obvious benefits to each of these strategies, the first challenge engineers often face with the building envelope is accurately modeling its performance. The second and perhaps greater challenge is constructing the envelope elements to match the designer’s intent.

Problems and challenges

Radiant heat gain: A building’s exterior enclosure absorbs short-wave radiation from sunlight, converting that solar energy to heat in the envelope assembly. Short-wave radiant energy passes through the glazing into the space, warming the objects inside. Longer-wave radiation in the sunlight is absorbed by the glass itself, heating the glass surfaces and re-radiating that heat into the building. This phenomenon—the “greenhouse effect”—produces solar heat gain in buildings.

We can employ a number of strategies to effectively manage solar heat gain in design. One of the most popular of these strategies is to apply a coating to the glass to reduce solar gains and/or glare. An uncoated glass surface has a thermal emissivity of 0.84 to 0.91, meaning that it absorbs and re-radiates up to 91% of the radiant thermal energy to which it is exposed. By selectively reducing the emissivity of the glass surfaces, by reducing the amount of energy that is both absorbed and transmitted through the glass, we reduce the amount of solar heat gain through the glazing by as much as 75%.

Solar glare: The glass that brings natural light into the building can also create a problem with solar glare, when the glazing is oriented toward the sun. Sunlight that reaches work surfaces where visual tasks are performed (both direct and reflected sunlight) can reflect off those working surfaces and into the eyes of the occupant, interfering with the ability to see the task. The most effective way to control solar glare problems within the building is by blocking direct sunlight before it reaches the exterior face of the glazing. Architects have increasingly employed sun shading devices that are designed to block direct sunlight from entering glazing systems in a variety of solar orientations. The design of solar glare controls can affect the engineering of both air conditioning and lighting systems.

Conductive heat loss: Glazing itself adds to building loads and comfort challenges by increasing conductive heat gains and losses. And, while these heat gains are significant, the heat gain/loss through thermal bridging (interruptions or penetrations of the insulation barrier by structural elements of the building envelope) is one significant controllable element of the envelope system and the structures in which glazing is mounted.

For example, gaps in the exterior wall construction for insulation are specifically called for in many new school buildings constructed in the mid-Atlantic region. Where glazing is to be installed, concrete masonry walls are thickened to allow window framing to be secured to backup walls. Insulation is often omitted at these window openings, creating gaps in the thermal insulation barrier. Glazing systems often are deeply recessed into wall openings or are installed so that the glass is flush with the surrounding exterior wall material. Either design decision can create a condition in which the thermal plane of the glazing (the insulated glass) is not contiguous with the thermal barrier in the surrounding wall system. Laboratory testing has shown a decrease in thermal performance of up to 5% can result from gaps in the insulation coverage equal to 1% of the wall area.

Many solar shading devices are supported on the structural framework at the outer walls. Whether integral to or independent of a curtainwall system, such shading devices typically require structural support members to penetrate the exterior wall systems to enable direct attachment to building structural members. These connections create conductive heat losses via “thermal bridges” that pass through the insulation barrier. These thermal bridges are often seen in infrared images of interior and exterior envelopes.